Videocommunication equipment, such as videoconferencing systems and videophone devices, have enabled people to communicate visually without having to travel to a common location. As a result, communication participants can be separated by large distances.
A typical videoconferencing uses a video camera to capture a series of images of a target, such as a meeting participant or a document. The series of images is encoded as a data stream and transmitted over a communications channel to a remote location. For example, the data stream may be transmitted over a phone line, an integrated services digital network (ISDN) line, or the Internet. The encoding process is typically implemented using a digital video coder/decoder (codec), which divides the images into blocks and compresses the blocks according to a video compression standard, such as the H.263 and H.261 recommendations by the Telecommunication Standardization Sector of the International Telecommunication Union (ITU-T). In standards of this type, a block may be compressed independent of the previous image or as a difference between the block and part of the previous image.
In a typical videoconferencing system, the data stream is received at a remote location, where it is decoded into a series of images, which may be viewed at the remote location. Depending on the equipment used, this process typically occurs at a rate of one to thirty frames per second.
In some videoconferencing applications, it is desirable to transmit a high quality still image. Until the image is completely received and decoded, the receiving terminal is often unaware of its content. Some decoders decode and display a block only after they have received the complete image. With the image being transmitted as a series of blocks, considerable delay is often involved in transmitting the entire image. For example, in applications where the available bandwidth for transmitting data is small, transmission of a 352.times.288 pixel image may require up to a minute. In order to transmit still images more quickly, the image may be highly compressed.
The above-mentioned Telecommunication Standardization Sector recently revised ITU-T H.263 recommendation by appending thereto Annex J: Deblocking Filter Mode. This annex describes an optional loop filter (also referred to as block edge filter or deblocking filter) to be used within the prediction loop used for coding in each of the send and receive terminals in image communicating system. The main purpose of the loop filter is to reduce blocking artifacts. Such artifacts often appear at boundaries between different image blocks. The above-mentioned annex, not unlike other recommendations by the ITU, was adopted after much research and consideration for the purpose of providing communicating image terminals of various types and manufacturers the ability to communicate images accurately.
Even without the deblocking filter problem introduced via the Annex J document, there can be divergence between local and remote terminals because of the potential for different, but Annex-A-compliant, IDCTs. To mitigate this problem, the standards require that a refresh intra-block be sent every for every transmitted set of coded blocks (for example, every 132 coded blocks). This is problematic, however, in that the bits required for a refresh block are often greater than the bits normally required to code the video data. This disadvantageous in that it requires extra bits for the transmission, and the resulting block that is viewed at the receiving terminal is unduly distorted.